Lab Case 114 – Interpretation

75 year old female with shortness of breath and tachypnoea.

Her blood gas shows:

Mild tachypnoea with respiratory alkalosis

partial metabolic compensation, possible additional metabolic acidodis

hypoxia (low O2)

Chronic hyperventilation with acute exacerbation. The patient is working hard but achieving inadequate oxygenation ie hypoxia despite additional respiratory effort.

Mild hyponatraemia and hypochloraemia

Large A-a gradient (50, cheat for normal value = Age/4 + 4):

  1. Diffusion defect
  2. V/Q mismatch
  3. Right-to-Left shunt (intrapulmonary or cardiac)
  4. Increased O2 extraction (CaO2-CvO2)

Causes of Increased A-a Gradient:

  1. Right to Left Intrapulmonary Shunt (due to fluid filled alveoli)
    1. CCF
    2. ARDS
    3. Lobar Pneumonia
  2. V/Q Mismatch (due to lung dead space)
    1. PE
    2. Atelectasis
    3. Pneumonia
    4. Obstructive lung disease (e.g. Asthma, COPD)
    5. Pneumothorax
  3. Alveolar hypoventilation
    1. Interstitial lung disease
  4. Increased oxygen extraction:
    1. Inadequate O2 delivery
  • hypoxic hypoxia: (low FiO2 gas or high altitude; lung disease)
  • hemoglobin (anemia)
  • contractility
  • rate/ rhythm
  • afterload
  • preload
  • shock/ hypoperfusion due to other causes

B. Increased oxygen consumption (VO2)

  • fever and inflammatory states, e.g. sepsis, burns, trauma, surgery
  • increased metabolic rate, e.g. hyperthyroidism, adrenergic drugs, hyperthermia, burns
  • increased muscular activity, e.g. exercise, shivering, seizures, agitation/anxiety/pain, weaning from ventilation/ increased respiratory effort

Causes of Normal A-a gradient:

  1. Hypoventilation
    1. Neuromuscular disorders
    2. Central nervous system disorder
  2. Low inspired FIO2 (e.g. high altitude)

Causes of low Oxygen extraction:

Increased oxygen delivery

  • hyperoxia, e.g high FiO2 gas, hyperbaric oxygen or ECMO

Decreased oxygen consumption

  • decreased metabolic rate, e.g. hypothyroidism, sedatives/ hypnotics, hypothermia
  • decreased muscular activity e.g. sedation/analgesics, muscle paralysis, ventilatory support
  • antipyretics
  • Starvation/hyponutrition
  • Sepsis due to shunting and histotoxic hypoxia
  • Histotoxic hypoxia, e.g. cyanide poisoning